A screen capture from a video of North Korea’s Unha-3 rocket lifting off from the Sohae launching station in Tongchang-ri, North Korea in December 2012. (Reuters)

A US think tank, using satellite imagery, says North Korea may be preparing to conduct a fifth nuclear test in the near future.

The satellite images show vehicle movement at a North site “not often seen there except during preparations for a [nuclear] test,” according to the 38 North website, a blog run by the Johns Hopkins University’s School of Advanced International Studies in Washington, DC.

The analysis from 38 North said the imagery of the Punggye-ri test site “suggests that Pyongyang may be preparing for a nuclear test in the near future.”

There has been intense speculation that Pyongyang may carry out its fifth test of a nuclear weapon during a congress of the ruling Workers’ Party now underway.

A senior US official, asked about the 38 North report, repeated a call for North Korea “to refrain from actions and rhetoric that further destabilize the region” and stick to past commitments to denuclearize.

“Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad,” he added. “We are prepared to defend ourselves and our allies from any attack or provocation.”

The 38 North website reported last month that satellite images showed North Korea may have resumed tunneling at Punggye-ri, activity similar to that seen before the country’s most recent nuclear test in January.

The website reported in December that satellite photographs from the two previous months indicated North Korea was digging a new tunnel for nuclear testing.

North Korea declared itself a nuclear power in 2005 and carried out four nuclear weapons tests in 2006, 2009, 2013 and 2016.

North Korea has vowed to conduct more nuclear testing in spite of stepped-up international sanctions after its previous test and a subsequent space rocket launch seen as a cover for development of its intercontinental ballistic missile program.

Pyongyang accuses the US of plotting with regional allies to topple its government, and says it will not relinquish its nuclear deterrence unless Washington ends its hostile policy toward Pyongyang and dissolves the US-led UN command in South Korea.